This invention relates to the conversion of asbestos cement products (which are considered to be harmful because of their content of asbestos fibre) into a harmless product, namely a product that is substantially free of asbestos fibres and so is considered to be inherently much less harmful than the starting asbestos cement product. This harmless product can be, for instance, an aggregate or a product comprising synthetic mineral fibres, which can alternatively be named as Man Made Vitreous Fibres (MMV fibres).
The health hazards which are associated with asbestos-containing products have resulted in a ban on the use of such products in new constructions and in the need to find environmentally acceptable ways of disposing of existing asbestos-containing products.
These problems are associated both with products having a very high content of asbestos fibre, such as asbestos insulation and other fibrous material and products having lower contents of asbestos such as asbestos cement products.
One way of attempting to dispose of asbestos-containing products is by dumping them at special sites under conditions such that asbestos fibres cannot subsequently escape, for instance by wrapping the products in plastic foil. This is particularly inconvenient for rigid, bulky products such as asbestos cement products.
There have been several proposals in the literature for rendering asbestos products harmless by heating the products.
In EP-A-344563 chrysotile asbestos is heated at a temperature of at least 580.degree. C. (eg. up to 650.degree. C.) so as to eliminate crystal-bonded water and convert the fibrous chrysotile asbestos into non-fibrous mineral forsterite.
In EP-A-265051 it is proposed to heat fibrous asbestos in an induction furnace at a temperature preferably between 1400.degree. and 1500.degree. C. in the absence of any additives, and the melt is then discharged from the furnace and the product can be used in the ceramic or glass industries or as an adhesive or as a land fill. Although it is mentioned in EP265051 that asbestos can occur as an asbestos cement product, normal asbestos cement has a melting point in the range 1800.degree. C. to 2000.degree. C. or more and so clearly asbestos cement cannot be used in the described process of EP-A-265051.
In DE-A-4211161 asbestos products are rendered harmless by heating in a non-aqueous system at a temperature of not more than 1300.degree. C. The asbestos is subjected to the heating in the form of a blend with various additives. In some of the examples the asbestos is introduced as asbestos fibre and the blend is stated to melt during the heating at below 1300.degree. C. In other examples, the asbestos is introduced as a minor component (for instance 33 to 42% based on the dry weight) of a blend of asbestos cement and additives, and the heating is said to result in sintering. The combination of the asbestos cement and the amount and type of additives would not cause melting at the exemplified temperatures (eg. 1000.degree. C.).
In U.S. Pat. No. 4,820,328 asbestos is converted into a glass by adding asbestos to a melt in an electric glass melting furnace in a particular way, at a temperature of at least 1000.degree. C. It is stated that the preferred temperature is 350.degree. C. to 1380.degree. C. corresponding to a temperature of about 250.degree. C. at the top of the molten glass.
Although it is stated that 100% blue asbestos can be melted, preferably the asbestos is produced as a blend with glass cullet (waste glass) and usually also with caustic soda. In the examples, temperatures ranging from 1050.degree. C. to 1380.degree. C. are mentioned. It is stated that a blend of 78% asbestos and 22% cullet could be melted; the data in example 1 clearly shows the need to use large amounts of cullet, and usually also caustic soda, if satisfactory melting is to be obtained in that example. Thus the example shows that none of the blends that contain 50% asbestos or more provide a satisfactory melt. Instead, satisfactory melts are obtained only when asbestos is in an amount of 40% or less and caustic soda is present in an amount of at least 10%.
There is a reference in U.S. Pat. No. 4,820,328 to the fact that the asbestos that can be used in the invention can include asbestos cement materials. However it is clear that simple replacement of the asbestos fibre in the examples with asbestos cement would not be possible because asbestos cement has a melting point (1800.degree. C. to 2000.degree. C. or more) very much higher than any of the melting temperatures mentioned in U.S. Pat. No. 4,820,328.
Because of the very large amounts of asbestos cement boards, roofing, piping and other products that are having to be disposed of, there is an acute need of developing a cost effective and efficient way of rendering such products harmless, and which is more environmentally friendly than merely wrapping the asbestos cement in plastic and dumping it. No economic way has previously been proposed. As shown above, the literature is misleading in suggesting (erroneously) that it is possible to melt asbestos cement under conditions similar to those proposed for melting a charge consisting of asbestos fibre or asbestos fibre with glass.
Although melting asbestos cement by itself would, in theory, render it harmless, in practice this is not a viable process. The melting would require very high temperatures (at least 1800.degree. C.) and the melt would be highly corrosive because of the high calcium content of the asbestos cement. It is not practicable, in a cost effective process, to provide a furnace that can withstand this combination of temperature and corrosiveness.